This invention relates to a method that switches a heat pump into an opposite mode of operation at shutdown to eliminate un-powered reverse rotation.
Refrigerant systems are utilized to control the temperature and humidity of air in various indoor environments to be conditioned. In a typical refrigerant system operating in the cooling mode, a refrigerant is compressed in a compressor and delivered to a condenser (or outdoor heat exchanger in this case). In the condenser, heat is exchanged between outside ambient air and the refrigerant. From the condenser, the refrigerant passes to an expansion device, at which the refrigerant is expanded to a lower pressure and temperature, and then to an evaporator (or indoor heat exchanger). In the evaporator, heat is exchanged between the refrigerant and the indoor air, to condition the indoor air. When the refrigerant system is operating, the evaporator cools the air that is being supplied to the indoor environment.
The above description is of a refrigerant system being utilized in a cooling mode of operation. In the heating mode, the refrigerant flow through the system is essentially reversed. The indoor heat exchanger becomes the condenser and releases heat into the environment to be conditioned (heated in this case) and the outdoor heat exchanger serves the purpose of the evaporator and exchangers heat with a relatively cold outdoor air. Heat pumps are known as the systems that can reverse the refrigerant flow through the refrigerant cycle in order to operate in both heating and cooling modes. This is usually achieved by incorporating a four-way reversing valve or an equivalent device into the system schematic downstream of the compressor discharge port. The four-way reversing valve selectively directs the refrigerant flow through the indoor or outdoor heat exchanger when the system is in the heating or cooling mode of operation respectively. Furthermore, if the expansion device cannot handle the reversed flow, than a pair of expansion devices, each along with a check valve, are employed instead.
A problem known as “unpowered reverse rotation” can occur with certain types of compressors at shutdown. With certain types of compressors, such as for example screw compressors or scroll compressors, the compressed refrigerant can move back inwardly towards the compression chambers at shutdown. This refrigerant would re-expand causing compression elements to rotate in the reverse direction at high speed. This is undesirable, as it results in unwanted highly offensive noise, and can even cause potential damage to the compressor.
Discharge check valves have been incorporated into the compressor design to prevent this reverse flow of compressed refrigerant from entering compression chambers, however, these check valves are relatively expensive to incorporate into the compressor design, suffer from their own reliability problems, and thus have not always been successful in preventing reverse rotation. Consequently, it is desirable to prevent un-power reverse rotation, while eliminating installation of the check valve or adding redundancy if the check valve malfunctions.